Well even before getting into the military stuff we could mention some reasons:
1. Our history & culture, no matters what happens in case of war all Iranians would unite against the adversary. that is why Iran as oldest country that has remained throughout history.
2. The world economy, in case of war Iran could destroy the whole region oil industry which in comparison Saudi oil field attack would be considered as a joke.
3. Existence of israel, in 2006 it took 33 days in recent war 48 hours and just by one front.
4. Security of the EU, Iran is indeed a heavy anchor in whole regional stability mechanism.
5. Our military industry & equipment ,which unlike others whom buy their beautiful weapon, are mostly local and closed book for others ...
6. Our persevere & determination that we get what we want, american sanctions would cause any country collapse.